There is an increasing use of mass spectrometers for obtaining both qualitative and quantitative information, such as molecular weight, isotope ratios, and elucidation of structure, from a given sample. Recent developments in mass spectrometry ionization techniques enable the efficient producticn of both positive and negative ions. More useful analytical information can be obtained from detecting both the positive and negative ions produced by such techniques than that obtained when only positive ions are detected.
Conventional technology for detecting positive ions utilizes a dynode electron multiplier having a negatively charged cathode and its anode at ground potential. This technique offers substantial advantages of positive ion detector system simplicity, since the detector signal is generated at ground potential and thus can be easily massaged by conventional circuitry. The sensitivity of this prior art positive ion detector is, however, limited, and more refined mass spectrometry analysis could be obtained if the positive ion detector system sensitivity were increased.
Prior art techniques for detecting negative ions present complex problems not encountered with positive ion detectors. A standard negative ion detector technique places the cathode of the electron multiplier at a high positive voltage, e.g., +1.5kV to +2.5kV, while the anode is placed at a still higher positive voltage, e.g., +3kV to +5kV. This technique obtains satisfactory detection sensitivity, but has a significant disadvantage since the signal is obtained at a high positive voltage rather than at ground level. Accordingly, this system requires complete electronic circuitry operating at the high positive voltage for amplilfying the detected signal and converting that signal to a representative signal at ground potential. In additional to the substantial expense of this high voltage electronic circuitry, this prior art system can be sensitive to background and microphonic noise.
Another technique for detecting negative ions is disclosed in U.S. Pat. No. 4,423,324. This technique utilizes a conversion dynode which functions as an ion reflector and converts the incoming negative ions to positive ions, which are then directed to an electron multiplier with its output at ground potential for detection. Since the detected signal is at ground potential, complex preamplification circuitry is not required. Losses at the conversion dynode are, however, significant, and accordingly this technique offers reduced ion detection efficiency. Although the conversion efficiency should increase as the detected mass units increase, the actual conversion efficiency for a mass range below 500 may be only a few percent.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for detecting negative ions. More particularly, the concepts of the present invention allow for high sensitivity detection of negative ions from a quadrupole mass spectrometer without utilizing complex circuits for dealing with high voltage signals from the detector.